In recent years, the signal band of information communication equipment, such as a Personal Handyphone System (PHS) phone, a cellular phone and the like has reached a GHz band, and the CPU clock time of a computer has become high frequency. Since the dielectric loss of an electrical signal is proportional to a product a square root of a specific inductive capacity and a dielectric dissipation factor of an insulating layer which forms a circuit, and a frequency an electrical signal, the dielectric loss becomes larger when a frequency of a used signal is higher. On the other hand, increase of the dielectric loss attenuates an electrical signal and degrades the reliability of the signal, and in order to such degradation, it is important to select a material for an insulation layer which material has a small dielectric constant and a dielectric dissipation factor.
Removal of a polar group in a molecular structure of a material is effective for the reduction of the dielectric constant and/or the reduction of the dielectric dissipation factor, various materials have been proposed. Among them, the fluoro-resins represented by especially polytetrafluoroethylene (PTFE) are widely used as insulating layers of various electrical components which use a high frequency signal since they have a dielectric constant and a dielectric dissipation factor both of which are small. However, PTFE has restrictions as to its workability, adhesion property and a price, and therefore a more versatile material is desired. Especially, in addition to the above-mentioned dielectric characteristics, thinning of an insulation layer is required in an electronic component, such as an antenna circuit for RF-ID, and a flexible flat cable.
The former antenna circuit for RF-ID is a high frequency circuit used for an IC chip embedded card or tag for non-contact authentication, and is an electronic apparatus or an electronic device which delivers and receives a signal with its outside by wireless. According to the manner with which a signal is transferred, an operating frequency and a form of the card or the tag, an antenna circuit is formed on a plastic film.
There is are two main manners with which signals are delivered and received as mentioned above, one is an “electromagnetic induction manner” and the other is a “microwave manner.” The antenna of the former manner is a loop coiled form, and since communication is carried out by transform a magnetic field into an electric power, electromagnetic induction manner is referred to. The frequency band used for the RF tag of an electromagnetic induction manner is a 135 kHz or less band or a 13.56 MHz band, and the communication frequency is low. Therefore, in the antenna circuit of the electromagnetic induction manner, the low dielectric dissipation factor as to the insulation material is not so desired.
On the other hand, although the “microwave manner” does not have a transmitter of itself, a signal which impinges on the antenna of the tag from a reader is used wherein the received signal is modulated according to data of the tag, and thus modulated signal is simultaneously returned to the to the reader. This communication manner is also called the down spread modulation manner, and follows the principle of a radar. A UHF band (800 MHz to 1 GHz) or a 2.45 GHz band is used as a communication frequency band. A dipole antenna is used for the tag which antenna is designed according to a length of ½ to ¼ of the wavelength. The RF tag of the microwave manner has an advantage that a communication distance becomes longer compared with the tag of the electromagnetic induction manner, and it is widely and quickly used in a PD market etc. However, the frequency band used is high, and demand of the reduction in the dielectric dissipation factor as to the insulation material of the antenna circuit is very large.
The flexible flat cable is parallel arranged multi-core electric wire in the form of a thin tape structure in which both sides of rectangular conductors arranged parallel are sandwiched with plastic films, it is an electronic part which is indispensable to the miniaturization and the densification of a device as a lead wire for connecting printed circuit boards in an electronic parts or connecting a printed circuit board and an electronic part. The dielectric loss of the flexible flat cable will cause degradation of a transmitted signal, and error occurrence, which results in a reduction of the reliability of the electronic device.
However, the fluoro-resin has a restriction in a lower limit of the manufacturable thickness and its mechanical strength becomes insufficient upon thinning, a polyester is widely used as the insulation material in the above mentioned application since it is excellent in the mechanical property and able to b easily thinned. That is, no insulation material of the low dielectric loss is not put in practical use in those electronic parts.
It is noted that distributing voids (air) in an insulation material is also known as a manner to reduce the dielectric constant and/or the dielectric dissipation factor of the material. If this manner is adopted, an insulation material with a small dielectric constant can be obtained comparatively inexpensively. For example, it is disclosed that a perforated synthetic resin film is sandwiched by two sheets of synthetic resin films (see Patent Reference 1). Moreover, an electric/electronic device is disclosed of which insulation material contains fine hollow spheres (micro balloons of a glass, a silica, alumina or the like) such that its dielectric constant can become 2.5 or less, and an electric wire (magnet wire), a composite film, a varnish for coil impregnation, an adhesive tape, an insulation sleeve, and a prepreg material using such insulation material are also disclosed (see Patent Reference 2).    [Patent reference 1] Japanese Patent Unexamined Publication No. H09-151851    [Patent reference 2] Japanese Patent Unexamined Publication No. H11-288621
However, the above mentioned manners merely reduce the dielectric constant and the dielectric dissipation factor depending on a void fraction, and it has been said impossible to make an innovative dielectric loss. When foaming an insulation material which has a dielectric constant of ∈0 and a dielectric dissipation factor of tan δ0, an effective dielectric constant of ∈ and an effective dielectric dissipation factor of tan δ are known as Wagner's equation as the following formulas (1) and (2) provided that a foaming extent (a volume ratio of air voids to an original insulation material) is set to be P:
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            ɛ        =                                                            2                ⁢                                  ɛ                  0                                            +              1              -                              2                ⁢                                  P                  ⁡                                      (                                                                  ɛ                        0                                            -                      1                                        )                                                                                                      2                ⁢                                  ɛ                  0                                            +              1              +                              P                ⁡                                  (                                                            ɛ                      0                                        -                    1                                    )                                                              ⁢                      ɛ            0                                              (        1        )                                [                  Equation          ⁢                                          ⁢          2                ]                                                                      tan          ⁢                                          ⁢          δ                =                              tan            ⁢                                                  ⁢                          δ              0                                +                                    2              ⁢                              ɛ                0                            ⁢                                                          ⁢              tan              ⁢                                                          ⁢                                                δ                  0                                ⁡                                  (                                      1                    -                    P                                    )                                                                                    2                ⁢                                  ɛ                  0                                            +              1              -                              2                ⁢                                  P                  ⁡                                      (                                                                  ɛ                        0                                            -                      1                                        )                                                                                -                                                    ɛ                0                            ⁢              tan              ⁢                                                          ⁢                                                δ                  0                                ⁡                                  (                                      2                    +                    P                                    )                                                                                    2                ⁢                                  ɛ                  0                                            +              1              +                              P                ⁡                                  (                                                            ɛ                      0                                        -                    1                                    )                                                                                        (        2        )            
FIGS. 1 and 2 show the relationships between the foaming extent P and the effective dielectric constant ∈ and between the foaming extent P and the effective dielectric dissipation factor tan δ which are obtained according to said formula (1) and (2), respectively. It is seen from FIGS. 1 and 2 that when the foaming extent becomes larger, both of the effective dielectric constant also the effective dielectric dissipation factor becomes smaller. That is, the reduction of the dielectric constant and the reduction of the dielectric dissipation factor by the introduction of the voids which have been recognized up to now are only within the range predicted according to the above mentioned formula.
Moreover, although it is effective in the reduction in the dielectric constant, or the reduction in the dielectric dissipation factor to introduce voids into an insulation material, such introduction has been recognized up to now as a factor which reduces electrode performances of an insulation system. If an electrical voltage is applied to an insulation material in a solid state which has voids (air bubbles, voids) or gaps therein, air spaces having a lower dielectric breakdown strength will discharge firstly even with a low electrical voltage. If partial discharge like this is repeated, depressions (pits) of which ends have become sharp are present, and discharge will concentrate on the depressions, so that the electric field at the ends increases to cause the dendrite dielectric breakdown (tree dielectric breakdown). Once the tree (dendrite trace of the dielectric breakdown) is formed, an air space is made there, and it spreads longitudinally while the partial discharge there is involved. Finally, it results in the dielectric breakdown. That is, the introduction of the voids into the insulation material has been widely recognized as a cause of reducing the insulation performance up to now. When the thickness of an insulation layer becomes thinner, the insulation performance degradation becomes more problematic.
It is noted that using, as an insulation material, a biaxially stretched film which contains voids is disclosed as an example of the application to motor insulations for compressors (see Patent References 3, 4, and 5).    [Patent Reference 3] Japanese Patent Unexamined Publication No. H09-286867    [Patent Reference 4] Japanese Patent Unexamined Publication No. H11-25757    [Patent Reference 5] Japanese Patent Unexamined Publication No. 2000-169607
However, no effect of specific dielectric loss reduction has not been found out in any of those proposals. Patent reference 3 discloses that a void containing biaxially stretched polyester film having a specific gravity of 1.0 (apparent density is reduced by 29%) was used as an insulation material, a leakage current of a motor of a compressor was reduced by 23%. It can be easily understood from the above mentioned FIG. 1 that if 29% as the voids are introduced into the insulation, the dielectric loss is reduced by 23%. Moreover, Patent References 4 and 5 disclose that the dielectric constant of the insulation material falls according to the void content.
Moreover, an antenna circuit using a void containing film of which void content is t5 to 50% by volume, and which has been at least uniaxially stretched and oriented is disclosed (see Patent Reference 6).    [Patent Reference 6] Japanese Patent Unexamined Patent Publication No. 2005-100371
However, the invention disclosed in Patent Reference 6 does not correspond to the high frequency electronic parts defined according to the present invention, since it relates to a loop antenna circuit using an electromagnetic induction of a low frequency in a 135 kHz or less band or a 13.56 MHz band. This is clear from the configuration (a flat coiled form antenna (80 mm×48 mm)) of the antenna disclosed in Example 1 of Patent Reference 6. Moreover, the purpose of the invention of Patent Reference 6 is to provide an excellently designed antenna sheet in which none of an IC chip, an antenna nor irregularities of a circuit appears on a card surface, and not to provide the reduction of the dielectric loss of the electronic parts.
Namely, as to the application of the void containing film to the insulation material, only the example of the application to the motor insulation for the compressor and the example of the application to the antenna circuit in the less than 100 MHz electromagnetic induction manner are disclosed, and nothing about the outstanding property which is acquired for the first time when the film is applied to the high frequency circuit is neither disclosed nor suggested at all. Rather, the manner of forming the voids in the insulation material has been recognized to providing its insulation performance degradation, and thus it is a fact that it has been avoided.
Furthermore, as to the insulation layer of a high frequency circuit, the dielectric constant of an insulating layer and the dielectric dissipation factor may simultaneously be required because of the demands of formation of a delay circuit, impedance matching of wiring substrates in a low impedance circuit, miniaturization of a circuit pattern, and formation of a composite circuit wherein a capacitor is built in a substrate. Therefore, examples of electronic parts in which a high dielectric constant layer and a low dielectric constant layer are composed (see Patent References 7, 8 and 9).    [Patent Reference 7] Japanese Patent Unexamined Publication No. 2000-91717    [Patent Reference 8] Japanese Patent Unexamined Publication No. 2001-247733    [Patent Reference 9] Japanese Patent Unexamined Publication No. 2001-345212
Among those References, Patent Reference 7 discloses an example of an electronic part in which an insulating layer having a high dielectric constant and a low dielectric dissipation factor is used. In Patent References 8 and 9, an insulation layer having a high dielectric constant and a low dielectric dissipation factor is formed in the above mentioned insulating layer having a low dielectric constant and a low dielectric dissipation factor by distributing a high dielectric insulation material such as ceramic powder or metal powder subjected to an insulating treatment. However, it is essentially very difficult to make the dielectric dissipation factor small while maintaining a high dielectric constant by the manner distributing a low dielectric constant material such as voids (air) in the above mentioned insulation material.
As mentioned above, it is required to control the dielectric constant of an insulation layer in a high frequency electronic part according to a circuit to be formed. However, in any case, it is required to make an insulation layer to have a low dielectric dissipation factor in order to reduce the dielectric loss, and to avoid degradation of the insulation performance accompanied by the reduction of the dielectric dissipation factor.